Urban Sewer System Is an Incubator of Sewage-Derived Mercury-Methylating Microorganisms: A New Public Health Challenge

The release of sewage-derived Hg-methylating (hgcA) microorganisms is found to be the main contributor
to global surges
of MeHg levels in urban rivers, posing significant risks to ecosystems
and public health. However, the sources and drivers of these microorganisms
remain unclear. This study integrated global metagenomic analysis,
field sampling, and laboratory experiments to investigate their sources
and driving mechanisms in sewage. Large-scale public metagenomic analyses
revealed that external microbial sources, such as drinking water and
feces, contribute negligibly to Hg-methylating communities in sewage.
Consistently, in our field sampling, sewage discharged directly from
building drainage stacksprior to entering sewer systems
contained almost no detectable hgcA genes. However,
as sewage flows through sewer systems, hgcA gene
abundance increased by 19- to 58-fold, with sequences identical to
those in sewer biofilms, suggesting biofilm detachment as the primary
source. Laboratory-scale sewer reactor experiments further demonstrated
that biofilm maturation was accompanied by an increased hgcA abundance and elevated MeHg production rates. Metabolic profiling
of hgcA-bearing MAGs revealed their key adaptive
strategies in sewer environments. These findings suggest that urban
sewers unexpectedly act as incubators for Hg-methylators, facilitating
their prevalence in sewage and increasing the risk of downstream MeHg
production. This finding underscores the urgent need to optimize sewer
infrastructure and management strategies to address this emerging
public health threat.